Bulimia nervosa and binge eating disorder (binge eating disorders) are serious public health problems, in part because people with these disorders tend also to suffer other medical complications, such as obesity and depression. Recently, animal models of binge eating have been developed, in which rats are provided intermittent access to sweet and/or high fat food. As consumption of this food escalates over several weeks of access, neurochemical changes in the nucleus accumbens (NAc) are observed, including increases in expression of opioid receptors. Pilot experiments show that injection of a broad-spectrum opioid receptor antagonist into the NAc has more pronounced effects on sweet/fat liquid consumption in binge eating than in control rats. These results suggest that binge eating could be due, at least in part, to upregulation of opioidergic neurotransmission in the NAc. To test this hypothesis, we will first determine which of three opioid receptors (mu, delta and kappa) in the NAc contributes to consumption of palatable liquid, and whether these contributions are different in binge eating and control rats. We will also determine whether these contributions are specific to the core or shell regions of the NAc. In addition, we will determine whether encoding of palatability by NAc neurons differs from controls in binge eating rats. Finally, we will test the hypotheses that endogenous opioids contribute to palatability encoding by NAc neurons, and that this contribution is different in binge eating vs control animals. Our goal is to elucidate the neural mechanisms that underlie binge eating, so that pharmaceutical treatments for binge eating disorders can be developed that specifically target these mechanisms. Because the same neural circuits are involved in drug addiction, our studies will also contribute towards understanding the neural mechanisms of addiction, and of reward-seeking behavior in general.